This invention relates to microcircuit packages and more particularly to all-metal flat packages for microcircuits.
All-metal flat packages normally include three major metal components. The first component is a frame which generally is a continuous ring of metal that extends around the periphery of the package and which forms the side walls of the package. Electrical leads extending through the frame constitute the second component of the flat package. Most often the leads pass through two opposing sides of the package and are normally glass-sealed within holes in the frame. The third component of the package is the bottom upon which the microcircuit substrate is affixed. In addition to these three components, there is, of course, a lid which is attached after the microcircuit has been installed in the package.
Normally, all-metal microcircuit packages which are flat packages are made of Kovar, with the Kovar frame and bottom often being joined together by a high temperature copper braze Kovar is a trademark of the Westinghouse Corporation for an iron-nickel-cobalt alloy (29% nickel, 17% cobalt, 53% iron, 1% minor ingredients). Indeed Kovar flat packages comprise a great majority of the all-metal flat packages made in the world. Kovar is the likely choice for making all-metal flat packages since leads readily can be glass sealed in Kovar and since its coefficient of thermal expansion closely matches 96% alumina, the material normally used for microcircuit substrates which are housed in the packages. Since the coefficient of expansion of Kovar closely approximates that of alumina, the alumina substrate may be soldered to the Kovar package.
While Kovar has good glass-sealing and thermal expansion properties, it has a very low coefficient of thermal conductivity, about 0.04 Cal/Cm.sup.2 /CM/Sec/.degree.C. This presents a problem of heat dissipation from power chips through the bottom of the flat package to a heat sink. About the only practical way to reduce the thermal impedence of an all-Kovar flat package is to employ a thin bottom, thereby reducing the length of the heat path from the substrate of the microcircuit chip to the heat sink. Such technique, however, reduces the strength of the package bottom.